The prior art prosthesis utilizing a below-knee silicone liner is described in detail in various prosthetic journals. In 1989, C. H. Fillauer, CPO, C. H. Pritham, CPO, and K. D. Fillauer, CPO, reported on "The Evolution and Development of the Silicone Suction Socket (3S) for Below-Knee Prostheses" in the Journal of Prosthetics and Orthotics. In 1995, L. J. Haberman, CPO, R. A. Bedotto, CPO, LPT, and E. J. Colodney, MD, described the use of an injection-molded silicone liner with suspension tabs for below-knee amputees in the article entitled "Silicone Only Suspension" (SOS) in the Journal of Prosthetics and Orthotics.
The prior art below-knee silicone designs are constructed either of fabric impregnated with silicone or with room temperature vulcanized (RTV) silicone. The fabric-type liner is conventionally laminated with a liquid silicone (RTV) under a vacuum-type system. These liners have the disadvantage that they are porous and harbor and promote the growth of bacteria. Further, the fabric silicone liner lacks durability when the thin silicone material is subject to abrasion during daily use since it rubs against the residuum (stump) which eventually uncovers the underlying fabric which can irritate the skin.
Other RTV silicone liners have been disclosed in the prior art which are custom injection-molded. While these silicone liners are relatively non-porous and cannot harbor bacteria, they are inclined to be fragile, reducing their service life, and are also time-consuming to fabricate.
Pre-fabricated injection-molded silicone liners are now available to the amputee in various sizes and styles. They are more durable as they are constructed of a heat-cured, two part silicone composition. They are constructed free of a fabric matrix and, thus, will not irritate the skin as the inner surface is rubbed away. All of these commercially available silicone liners are, however, generally tubular in shape, offering very little tapering towards the distal end. Such silicone liners are in a straight-line orientation (windsock shaped) and respond poorly to angular (flexion) changes to the knee. As an illustration, significant wrinkling of these straight-line silicone liners occur in the back of the knee when the user sits down. Heightened compression of the kneecap (patella) occurs which can irritate the skin overlying the kneecap. Straight-line oriented liners also tend to bias the knee joint to a straight position when the knee is flexed, i.e., the liner is under tension and it wants to restore itself to its straight molded position. Thus, the amputee must maintain a flexure pressure in order to keep the knee and prosthesis from extending to its straight position.
Silicone liners typically utilize a hard, unyielding material, such as aluminum or polycarbonate as a shuttle screw housing (hereafter "umbrella") for affixing the lower leg portion of the prosthesis. Rigid aluminum shuttle screw housings and rigid polycarbonate shuttle screw housings are not flexible and have limitations as to functional life and/or comfort to the amputee. Polycarbonate umbrellas may fracture and fail within months of initial use. Aluminum umbrellas have no flexibility and cannot distort to comply with the naturally occurring movement of the residuum within the prosthetic socket.
Prior art silicone liners also lack anatomical symmetry to the amputation limb which results in marked distortion of the silicone liner during naturally occurring changes in the position of the knee and residuum. The lack of anatomical symmetry results in a reduction of the naturally occurring range of motion (ROM) present in the human knee due to the wrinkling and bunching of the silicone material in that region. The lack of anatomical symmetry causes an uneven stretching of the silicone liner which may be harmful to the underlying tissues, be uncomfortable to wear, and can result in premature wearing and subsequent tearing of the silicone liner. Prior art silicone liners lack reinforcement at their upper (proximal) edge which may result in premature tearing, requiring early replacement of the silicone liner.
Silicone liners also create conditions which increase the potential for growth of bacteria on the skin of the amputee. The silicone liner, which is a relatively impermeable material, tends to encourage the skin to be constantly moist from sweating. This condition creates the right environment for undesirable bacteria to proliferate on the residuum, thus making the amputee's skin susceptible to invasion by pathogenic microorganisms. The constant moisture present when wearing a silicone liner and the occlusion from the liner which occurs to the amputee's skin, increases the skin's vulnerability to bacteriological infection.